Method for producing a recombinant protein on a manufacturing scale

ABSTRACT

A method for producing a protein of interest on a manufacturing scale is based on integration, by homologous recombination, of the DNA encoding the protein of interest into a bacterial cell genome at a pre-selected site. The manufacturing scale production of recombinant proteins is in the fed-batch mode, semi-continuous or in a chemostat.

This application is a national phase entry under 35 U.S.C. 371 ofinternational application PCT/EP2008/056062, filed May 16, 2008, whichclaims priority to European Application No. EP 07009872.8, filed May 17,2007, each of which is hereby incorporated by reference in its entirety.

INTRODUCTION

The present invention relates to the production of recombinant proteinsin bacterial host cells on a manufacturing scale.

Currently, production of recombinant proteins in bacterial hosts, inparticular Escherichia coli, mostly uses plasmid-based expressionsystems. Since these systems provide high gene dosage and are wellestablished, they have become widely accepted, also because theavailable cloning protocols are simple to handle.

Plasmid-based expression is characterized by plasmid copy numbers up toseveral hundred per cell (Baneyx, 1999). Expression plasmids usuallycarry the gene of interest under the control of a promoter, an origin ofreplication (ori) and a marker gene for selection of plasmid-carryingclones. In addition, coding or non-coding or non-functional backbonesequences are frequently present on said plasmids (i.e. vectors). Thepresence of plasmids and the corresponding replication mechanism alterthe metabolism of the host cell (Diaz-Rizzi and Hernandez, 2000) andimpose a high metabolic burden on the cells, thereby limiting theirresources for recombinant protein production. In addition, theapplication of strong promoters in combination with high gene dosagetriggers a rate of recombinant protein formation that is usually toohigh for the host cell to cope with and may therefore lead to a quickand irreversible breakdown of the cell metabolism. Consequently, thehost cell's potential cannot be fully exploited in plasmid-basedsystems, resulting in low yield and quality of the recombinant protein.Thus, one of the major drawbacks of plasmid-based expression systems maybe attributed to the increased demand for nutrients and energy that isrequired for plasmid replication and maintenance.

Another typical phenomenon in plasmid-based systems is the change ofplasmid copy number in the course of cultivation. Recombinant proteinproduction is accompanied, at high expression rates, with starvation andcellular stress that lead to increased pools of uncharged tRNAs. Thisleads to an interference with the control mechanism of plasmid copynumber (PCN). Consequently, PCN increases rapidly and causes a breakdownof the cultivation process (so-called “run-away effect”). The run-awayphenomenon of ColE1 type plasmids after induction of recombinant geneexpression can lead to a strong increase of gene dosage (Grabherr etal., 2002).

Segregational instability, (i.e. the formation of plasmid free hostcells) and structural instability (i.e. mutations in plasmid sequence)are further problems often seen in plasmid-based systems. During celldivision, cells may lose the plasmid and, consequently, also the gene ofinterest. Such loss of plasmid depends on several external factors andincreases with the number of cell divisions (generations). This meansthat plasmid-based fermentations are limited with regard to the numberof generations or cell doublings (Summers, 1991).

Overall, due to these properties of plasmid-based expression systems,there is a limited yield of recombinant protein and a reducedcontrollability of process operation and process economics.Nevertheless, due to lack of more efficient alternatives, plasmid-basedbacterial expression systems became state of the art for production andisolation of heterologous recombinant proteins on a manufacturing scale.

Therefore, as an alternative to plasmid-based expression, genome-basedexpression systems have been explored. A well-known and widely-appliedexample for a heterologous protein that is chromosomally expressed in E.coli, is the RNA polymerase of the T7 phage, which serves the purpose ofplasmid-based transcription of a plasmid-based gene of interest. Thissystem, which is originally described in U.S. Pat. No. 4,952,496, isbased on non-site specific integration, and renders the resultingbacterial strains (e.g. E. coli BL21 (DE3) or HMS174 (DE3)) as lysogens.To prevent potential cell lysis followed by undesired phage release, theT7 polymerase gene was integrated into the chromosome without creatingphage lysogen, i.e. it was inserted by homologous recombination (WO2003/050240). Recently, integration of T7 RNA polymerase gene into thegenome of Corynebacterium acetoacidophilum has been described, again forthe purpose of plasmid-based expression of recombinant proteins (US2006/0003404).

Other methods for genomic integration of nucleic acid sequences—in whichrecombination is mediated by the Red recombinase function of the phage λ(Murphy, 1998) or the RecE/RecT recombinase function of the Rac prophage(Zhang et al., 1998)—have been suggested for protein expression studies,sequence insertions (e.g. of restriction sites, site-specificrecombinase target sites, protein tags, functional genes, promoters),deletions and substitutions (Muyrers et al., 2000).

WO 2001/18222 describes utilization of a chromosomal integration methodbased on the Saccharomyces cerevisiae FLP (flippase)/FRT (flippaserecognition target) recombination system (Pósfai et al., 1994) forinsertion of ethanol pathway genes from Zymomonas mobilis downstream ofchromosomal promoters of E. coli in order to confer ethanologenicproperties to that host. The FLP system was used for precisely removingsequences (markers and replicons) after chromosomal integration ofcircular vectors (Martinez-Morales et al., 1999).

A method developed by Datsenko and Wanner (2000) for insertion of linearDNA fragments with short homology sequences, utilizing the λ Redtechnology in combination with the FLP-based marker excision strategy,has been applied for the insertion of an antibiotic resistance gene forchromosomal gene replacement (Murphy, 1998) or for gene disruption(Datsenko and Wanner, 2000). Also, this method has been suggested foroverexpression of tagged homologous E. coli proteins by the chromosomalinsertion of a marker, a promoter and a His-tag prior to (i.e. upstreamof) said proteins (Jain, 2005).

Genome-based expression was also suggested for integration of arepressor molecule into the chromosome of E. coli to establish ahost/vector selection system for propagation of plasmids without anantibiotic resistance marker (WO 2006/029985). In the context of thatselection system, the genomic insertion of a reporter protein (greenfluorescent protein) was used as a model system to demonstrate aplasmid-derived RNAI/II antisense reaction (Pfaffenzeller et al., 2006).In a similar way, Zhou et al., (2004) described the integration of acircular vector carrying green fluorescent protein as a reportermolecule.

In WO 1996/40722, a method is described that makes use of integration ofa circular vector (so-called “circular chromosomal transfer DNA”, CTD)including a selectable marker into the bacterial chromosome (i.e. at theattB site of E. coli). In that method, by using duplicate DNA sequencesflanking the selection marker, amplification of the chromosomal genedosage was achieved. Thereby, the obtained chromosomal gene dosage wasapproximately 15-40 copies per cell, which is similar to those achievedby commonly used plasmid vectors. Cultivation of clones containingchromosomal transfer DNA integrated into the bacterial genome resultedin levels of recombinant proteins similar to those obtained byplasmid-based systems (Olson et al., 1998). This method requires invitro ligation of CTD and is, with regard to integration, limited to theattB site.

BRIEF DESCRIPTION OF INVENTION

It was an object of the invention to provide a novel manufacturing scaleexpression method for producing recombinant proteins.

The present invention relates to a method for producing a recombinantprotein of interest on a manufacturing scale, comprising the steps of

-   -   a) cultivating a population of bacterial expression host cells,    -   b) harvesting the protein of interest, and    -   c) isolating and purifying it, wherein        in step a), cultivation is in a mode that employs the addition        of a feed medium, said mode being selected from the fed-batch,        semi-continuos or continuous mode, and wherein said bacterial        expression host cells contain, integrated in their genome, a DNA        construct carrying the DNA sequence encoding the protein of        interest under the control of a promoter that enables expression        of said protein.

In the following, the term “host cell” designates the bacterial cellthat is used as the starting cell prior to its transformation with theDNA construct carrying the gene of interest. The cell that has, upontransformation, the linear DNA construct that carries the gene ofinterest, integrated in its genome, is termed “expression host cell”.

The expression cassette contains, as its essential elements, the gene ofinterest, a promoter and a ribosome binding site (RBS). Additionally,the expression cassette may contain sequences encoding “helperproteins”, e.g. markers or proteins that favour growth of the expressionhost cell, e.g. genes relevant for sugar or protein metabolism and/orproteins that improve expression of the gene of interest, e.g. chaperonmolecules (like GroEl or GroEs) or genes that are involved intranscription and translation.

DETAILED DESCRIPTION OF THE INVENTION

Protein of Interest

With regard to the protein of interest, there are no limitations. Itmay, in principal, be any protein that is to be produced on amanufacturing scale, e.g. an industrial protein or a therapeuticprotein. Examples for proteins that can be produced by the method of theinvention are, without limitation, enzymes, regulatory proteins,receptors, peptides, e.g. peptide hormones, cytokines, membrane ortransport proteins. The proteins of interest may also be antigens asused for vaccination, vaccines, antigen-binding proteins, immunestimulatory proteins, allergens, full-length antibodies or antibodyfragments or derivatives. Antibody derivatives may be selected from thegroup of single chain antibodies, (scF_(V)), Fab fragments, F_(V)fragments, single domain antibodies (V_(H) or V_(L) fragment), domainantibodies like camelid single domain antibodies (V_(HH), nanobodies) orother antibody formats as described for instance in Andersen and Reilly(2004) or Holliger and Hudson (2005). The DNA molecule encoding theprotein of interest is also termed “gene of interest”.

Promoter

“Promoter” in the meaning of the present invention is an expressioncontrol element that permits binding of RNA polymerase and theinitiation of transcription. In one embodiment of the invention, thegene of interest is under the control of a “strong” promoter. A strongpromoter is characterized by a high binding affinity of the promotersequence to an RNA polymerase, usually the naturally occurringcorresponding RNA polymerase, on the one hand and the rate of formationof mRNA by that RNA polymerase on the other hand.

Preferably, the gene of interest is under the control of an induciblepromoter. An inducible promoter is a promoter that is regulable byexternal factors, e.g. the presence of an inductor (also termed“inducer”) molecule or the absence of a repressor molecule, or physicalfactors like increased or decreased temperature, osmolarity, or pHvalue. Different promoters and the respective induction principles werereviewed by Makrides (1996).

In the present invention, promoters may be used that have been developedfor plasmid-based expression, among the strong promoters, the T7promoter of bacteriophage T7 has been most widely used.

In a preferred aspect, the promoter is the T7 promoter. T7-basedexpression systems, comprising a combination of the T7 promoter and T7polymerase gene under the control of the lac promoter, are widely usedfor large scale expression of recombinant proteins in both bacterial andeukaryotic cells. The system makes use of the T7 RNA polymerase,transcription rate of which is several times higher than E. coli RNApolymerase. The expression from the T7 promoter is under the control ofT7 RNA polymerase, which is stringently specific for the T7 promoter(Chamberlin et al., 1970), i.e. the T7 promoter can only be utilized bythe polymerase of bacteriophage T7. When IPTG is added to the culturemedium, T7 RNA polymerase is expressed by transcription from the lacpromoter.

In the meaning of the present invention, the term “T7 promoter” includespromoters that are present in the genome of bacteriophage T7, as well asconsensus sequences and variants of such promoters with the ability tomediate transcription by the T7 RNA polymerase. The bacteriophage T7contains seventeen different promoter sequences, all of which comprise ahighly conserved nucleotide sequence (Oakley and Coleman, 1977;Panayotatos and Wells, 1979).

According to certain embodiments of the invention, the gene of interestmay be under the control of the tac or the trc promoter, the lac or thelacUV5 promoter (all inducible by lactose or its analogue IPTG(isopropylthiol-β-D-galactoside)), the tightly regulatable araBADpromoter (P_(BAD); Guzman et al., 1995, inducible by arabinose), the trppromoter (inducible by β-indole acrylic acid addition or tryptophanstarvation, repressible by tryptophan addition), the lambda promoter pL(λ) (induction by an increase of temperature), the phoA promoter(inducible by phosphate starvation) or other promoters suitable forrecombinant protein expression (Makrides, 1996), which all use E. coliRNA polymerase.

Among inducible promoters are those that show a “leaky” expressionbehaviour. Such promoters (so-called “leaky promoters”) are, inprinciple, inducible, but show nevertheless also basal expressionwithout being externally induced. Inducible promoters that show leakyexpression under non-induced conditions may behave similarly toconstitutive promoters (i.e. they are steadily and continuously activeor they may be activated or enhanced as a result of certain cultivationconditions). Leaky promoters may be particularly useful for continuouslyoperated cultivation processes. Examples are the T7 promoter and the trppromoter.

In the method of the invention, the promoter may also be constitutive,i.e. a promoter which controls expression without the need for inductionon the one hand, or the possibility of repression on the other hand.Hence, there is continuous and steady expression at a certain level.Aside the advantage that they do not require an inducer, constitutivepromoters are particularly useful in embodiments where the method iscontinuous (as described below). As an example, the strong constitutiveHCD promoter (Poo et al., 2002; Jeong et al., 2004) may be applied forconstitutive expression.

The linear cassette integrated in the genome of the bacterial cells alsocontains, immediately upstream of the gene of interest, a Shine-Dalgarno(SD) sequence, also termed ribosome binding site (RBS).

Expression Cartridge

In the following, the linear or circular DNA construct to be integratedinto the bacterial genome is also termed “expression cartridge” or“cartridge”. As a result of integration, the expression host cell has anintegrated “expression cassette”. Preferably, the cartridge is a linearDNA construct comprising essentially a promoter, a gene of interest, aribosome binding site and two terminally flanking regions which arehomologous to a genomic region and which enable homologous recombination(see insertion cartridge shown in FIG. 10). In addition, the cartridgemay contain other sequences as described below in detail; e.g. sequencescoding for antibiotic selection markers, prototrophic selection markersor fluorescent markers, markers coding for a metabolic gene, genes whichimprove protein expression (like e.g. the T7 RNA polymerase gene) or twoflippase recognition target sites (FRT) which enable the removal ofcertain sequences (e.g. antibiotic resistance genes) after integration.

The cartridge is synthesized and amplified by methods known in the art,in the case of linear cartridges, usually by standard polymerase chainreaction, PCR (Wilson and Walker, 2005). Since linear cartridges areusually easier to construct, they are preferred for obtaining theexpression host cells used in the method of the invention. Moreover, theuse of a linear expression cartridge provides the advantage that thegenomic integration site can be freely chosen by the respective designof the flanking homologous regions of the cartridge. Thereby,integration of the linear expression cartridge allows for greatervariability with regard to the genomic region.

Host Cells

Regarding the bacterial host cells, there are, in principle, nolimitations; they may be eubacteria (gram-positive or gram-negative) orarchaebacteria, as long as they allow genetic manipulation for insertionof a gene of interest, advantageously for site-specific integration, andcan be cultivated on a manufacturing scale. Preferably, the host cellhas the property to allow cultivation to high cell densities.

Examples for bacterial host cells that have been shown to be suitablefor recombinant industrial protein production are Escherichia coli (Lee,1996; Hannig and Makrides, 1998), Bacillus subtilis, Pseudomonasfluorescens (Squires et al., 2004; Retallack et al., 2006) as well asvarious Corynebacterium (US 2006/0003404 A1) and Lactococcus lactis(Mierau et al., 2005) strains. Preferably, the host cells are E. colicells.

Another requirement to the host cell is that it contains an RNApolymerase that can bind to the promoter controlling the gene ofinterest. The RNA polymerase may be endogenous or foreign to the hostcell.

Preferably, host cells with a foreign strong RNA polymerase are used.Most preferably host strains, e.g. E. coli strains, are used that havebeen engineered to carry a foreign RNA polymerase (like e.g. in the caseof using a T7 promoter a T7-like RNA polymerase in the so-called “T7strains”) integrated in their genome. Examples for T7 strains are widelyused and commercially available, e.g. BL21 (DE3), HMS174 (DE3) and theirderivatives or relatives (Novagen, pET System manual, 11^(th) edition).These strains are DE3 lysogens containing the T7 RNA polymerase geneunder control of the lacUV5 promoter. Induction with IPTG allowsproduction of T7 RNA polymerase which then directs the expression of thegene of interest under the control of the T7 promoter.

Preferably, a host cell is used that contains a T7 RNA polymerase whichhas been genome-integrated without generating phage lysogens, e.g. asdescribed in WO 2003/050240. (Cells that do not generate phage lysogensare “non-lysogenic”).

A “T7-like RNA polymerase” includes, but is not limited to, RNApolymerases from other T7-like phages, such as the T3 RNA polymerase, asdisclosed in U.S. Pat. No. 4,952,496. A prerequisite of a T7-like RNApolymerase is that there should be a highly specific cognate promoter,and that the gene of interest is transcribed at a high rate in thepresence of said RNA polymerase.

Alternatively to using the above-mentioned strains carrying the T7 RNApolymerase, such strains can be generated de novo, e.g. as described inU.S. Pat. No. 4,952,496, US 2006/0003404, or WO 2003/050240.

The host cell strains E. coli BL21 (DE3) or HMS174 (DE3), which havereceived their genome-based T7 RNA polymerase via the phage DE3, arelysogenic. Lysogenic strains have the disadvantage to potentiallyexhibit lytic properties, leading to undesirable phage release and celllysis. For the invention, it is therefore preferred that the T7 RNApolymerase contained in the host cell has been integrated by a methodwhich avoids, or preferably excludes, the insertion of residual phagesequences in the host cell genome. In a preferred embodiment of theinvention, the host cell has been obtained by integrating the T7polymerase into the cell's genome by site-directed integration, e.g.according to the method described by Datsenko and Wanner (2000).

In the case of using the T7 system, a host cell may be used that has theT7 RNA polymerase in combination with its promoter (preferably thelacUV5 promoter) integrated in its genome.

Alternatively, the T7 RNA polymerase may be provided as an element ofthe expression cartridge and thus becomes integrated into the cell'sgenome as part of the expression cassette.

Another requirement to the host cell is its ability to carry outhomologous recombination (which is relevant for integration of theexpression cartridge into the genome and optionally for P1transduction).

Therefore, the host cell preferably carries the function of therecombination protein RecA. However, since RecA may cause undesirablerecombination events during cultivation, the host cell preferably has agenomic mutation in its genomic recA site (rendering it dysfunctional),but has instead the RecA function provided by a recA sequence present ona helper plasmid, which can be removed (cured) after recombination byutilizing the helper plasmid's temperature-sensitive replicon (Datsenkoand Wanner, 2000).

In view of recombination, in addition to RecA, the host cell preferablycontains, in its genome, DNA sequences encoding recombination proteins(e.g. Exo, Beta and Gam). In this case, a host cell may be selected thatalready has this feature, or a host cell is generated de novo by geneticengineering to insert these sequences.

In view of site-specific gene insertion, another requirement to the hostcell is that it contains at least one genomic region (either a coding orany non-coding functional or non-functional region or a region withunknown function) that is known by its sequence and that can bedisrupted or otherwise manipulated to allow insertion of a heterologoussequence, without being detrimental to the cell.

In certain embodiments, the host cell carries, in its genome, a markergene in view of selection.

Integration Locus

With regard to the integration locus, the expression system used in theinvention allows for a wide variability. In principle, any locus withknown sequence may be chosen, with the proviso that the function of thesequence is either dispensable or, if essential, can be complemented (ase.g. in the case of an auxotrophy).

When choosing the integration locus, it needs to be considered that themutation frequency of DNA caused by the so-called “adaptive evolution”(Herring et al, 2006) varies across the genome of E. coli and that themetabolic load triggered by chromosomally encoded recombinant geneexpression may cause an enhanced mutation frequency at the integrationsite. In order to obtain an expression host cell that is robust andstable, a highly conserved genomic region that results in a loweredmutation frequency is preferably selected as integration site. Suchhighly conserved regions of the E. coli genome are for instance thegenes encoding components of the ribosome or genes involved inpeptidoglycan biosynthesis (Mau et al., 2006), and those regions arepreferably selected for integration of the expression cartridge. Theexact integration locus is thereby selected in such a way thatfunctional genes are neither destroyed nor impaired, and the integrationsite should rather be located in non-functional regions.

The genomic region with known sequence that can be chosen forintegration of the cartridge may be selected from the coding region of anon-essential gene or a part thereof; from a dispensable non-codingfunctional region (i.e. promoter, transposon, etc.), from genes thedeletion of which may have advantageous effects in view of production ofa specific protein of interest, e.g. certain proteases, outer membraneproteins like OmpT, potential contaminants of the product, genesencoding proteins of metabolism (e.g. relevant for the metabolism of asugar molecule that is undesirable or dispensable for a given hoststrain and/or fermentation process) or stress signalling pathways, e.g.those occurring in stringent response, a translational control mechanismof prokaryotes that represses tRNA and rRNA synthesis during amino acidstarvation (Cashel, 1969).

Alternatively, the site of integration may be a marker gene which allowsselection for disappearance of said marker phenotype after integration.Preferably, a fluorescent marker like the green fluorescent protein, thedeletion of which leads to a disappearance of fluorescence, and whichallows visual detection of clones that carry the expression cassette, isused (FIG. 10).

Alternatively, the site useful to select for integration is a functionwhich, when deleted, provides an auxotrophy. In this case, theintegration site may an enzyme involved in biosynthesis or metabolicpathways, the deletion of such enzyme resulting in an auxotrophicstrain. Positive clones, i.e. those carrying the expression cassette,may be selected for auxotrophy for the substrate or precursor moleculesof said enzymes (FIG. 10).

Alternatively, the site of integration may be an auxotrophic marker (anon-functional, i.e. defective gene) which is replaced/complemented bythe corresponding prototrophic marker (i.e. a sequence that complementsor replaces the defective sequence) present on the expression cassette,thus allowing for prototrophic selection.

In one aspect, the region is a non-essential gene. According to oneaspect, this may be a gene that is per se non-essential for the cell.

Non-essential bacterial genes are known from the literature, e.g. fromGerdes et all. (2002 and 2003), from the PEC (Profiling of the E. coliChromosome) database or from the so-called “Keio collection” (Baba etal., 2006).

An example for a non-essential gene is RecA. Integrating the expressioncassette at this site provides the genomic mutation described above inthe context with the requirements on the host cells.

Suitable integration sites, e.g. sites that are easily accessible and/orare expected to yield higher expression rates, can be determined inpreliminary screens. Such screens can be performed by generating aseries of single mutant deletions according to the Keio collection (Babaet al., 2006) whereby the integration cartridge features, as variableelements, various recombination sequences that have been pre-selected inview of specific integration sites, and, as constant elements, the basicsequences for integration and selection, including, as a surrogate “geneof interest”, a DNA sequence encoding an easily detectable protein underthe control of an inducible promoter, e.g. the cartridge used in Example5, encoding the Green Fluorescent Protein. The expression level of thethus created single knockout mutants can be easily quantified byfluorescence measurement. Based on the results of this procedure, acustomized expression level of a desired target protein can be achievedby variation of the integration site and/or number of integratedcartridges.

In the embodiments in which the host cell contains DNA sequencesencoding recombination proteins (e.g. Exo, Beta and Gam) in itsgenome—either as a feature of the starting cell or obtained by geneticengineering—integration can occur at the genomic site where theserecombination protein sequences are located. By integration of theexpression cartridge, the sequences coding for the recombinationproteins are destroyed or removed and consequently need not, as in thecase of plasmid-encoded helper proteins, be removed in a separate step(FIG. 9).

Integration Methods

With regard to the integration locus, as described above, any locus withknown sequence may be chosen, with the proviso that the function of thesequence is either dispensable or, if essential, can be complemented. Ina preferred embodiment, integration of a linear cartridge is at anattachment site like the attB site or the attTn7 site, which arewell-proven integration sites. However, the flexibility of the systemallows the insertion of the gene of interest at any pre-selected locus,as described above.

Integration of the gene of interest into the bacterial genome can beachieved by conventional methods, e.g. by using linear cartridges thatcontain flanking sequences homologous to a specific site on thechromosome, as described for the attTn7-site (Rogers et al., 1986;Waddell and Craig, 1988; Craig, 1989). The cartridge is transformed intothe cells of an E. coli strain, e.g. E. coli MG1655, that contains theplasmid pKD46 (Datsenko and Wanner; 2000). This plasmid carries thephage λ derived Red function (γ.β. exo) that promotes recombination invivo.

Alternatively, an expression cassette can first be integrated into thegenome of an intermediate donor host cell, from which it can then betransferred to the host cell by transduction by the P1 phage (Sternbergand Hoess, 1983; and Lennox, 1955), the donor cell being e.g. MG1655 orDY378 (Yu et al., 2000), an E. coli K12 strain which carries thedefective λ prophage. In brief, P1 transduction is a method used to moveselectable genetic markers from one “donor” strain to another“recipient” strain. During the replication and lysis of the phage in aculture of bacteria, a small percentage of the phage particles willcontain a genome segment that contains the expression cassette. Once aphage population has been generated from a donor cell, the phage areused to infect the recipient cell, in the case of the invention, thehost cell. Most of the bacteria are lysed by phage that packaged P1genomes, but a fraction of the phage inject a genome segment derivedfrom the donor host. Homologous recombination then allows the incominggenomic segment to replace the existing homologous segment. The infectedrecipient bacteria are plated on a medium that selects for the genomesegment of the donor bacteria (antibiotic resistance, prototrophy,fluorescence marker, etc.). Importantly, the infectivity of the phagehas to be controlled. Otherwise, phage released from neighboring cellswould infect and lyse the bacteria that had been infected withtransducing particles. Phage P1 requires calcium for infectivity,therefore it can be controlled by growing in the presence and absence ofcalcium. The calcium chelator citrate is usually used because it lowersthe concentration of free calcium (by forming Ca-citrate) low enough toprevent P1 infection, but not so low as to starve the cells for calcium.

To simplify the integration procedure, as shown in FIG. 9, instead ofproviding the recombination proteins by means of the plasmid pKD20 orpKD46 according to Datsenko and Wanner (2000), the genes encoding therecombination functions exo, beta and gam may already be present in thechromosome of the host cell, the expression of recombination proteinsbeing controllable by inducible promoters, e.g. frt, lac, tac, T7 or thearaP promoter. By application of the corresponding inducer, the thusmodified strain can be used for integration of the linear expressioncartridge at the site of the recombination proteins. Thereby, therecombination proteins are first expressed and then utilized tointegrate an expression cartridge into the genomic region coding forsaid recombination proteins. Thereby, as a consequence of recombination,and as a particular advantage of this embodiment, the genes coding forthe recombination proteins are removed (excised) or destroyed (FIG. 9).Furthermore, this embodiment has the advantage that there is no need forplasmids encoding these recombination functions.

Examples, without limitation, of other integration methods useful in thepresent invention are e.g. those based on Red/ET recombination (Muyrerset al., 2002; Wenzel et al., 2005; Vetcher et al., 2005).

Selection Markers

Positive clones, i.e. clones that carry the expression cassette, can beselected by means of a marker gene.

In some embodiments, host cells are used that already contain a markergene integrated in their genome, e.g. an antibiotic resistance gene or agene encoding a fluorescent protein, e.g. GFP (as shown in FIG. 10). Inthis case, the expression cartridge which does not contain a selectionmarker, is integrated at the locus of the chromosomal marker gene, andpositive clones are selected for loss/disappearance of the respectivephenotype, e.g. they are selected for antibiotic sensitivity ordisappearance of fluorescence, which can be directly visualized on thecultivation plates. These embodiments have the advantage that the markeris either interrupted or completely replaced by the expression cassette,and thus no functional marker sequence is present after integration anddoes not need to be removed, if undesirable, as in the case ofantibiotic resistance genes.

Alternatively, the marker gene is part of the expression cartridge. Inthe case that the marker used for selection is a gene conferringantibiotic resistance (e.g. for kanamycin or chloramphenicol), positiveclones are selected for antibiotic resistance (i.e. growth in thepresence of the respective antibiotic), as shown in FIG. 7. Preferably,kanamycin is used for antibiotic clone selection.

The marker gene (irrespective of whether it is present on the hostcell's genome or has been introduced by means of the expressioncartridge) can be eliminated upon integration of the cassette, asschematically shown in FIG. 7, e.g. by using the FLP recombinasefunction based on the site-specific recombination system of the yeast 2micron plasmid, the FLP recombinase and its recombination target sitesFRTs (Datsenko and Wanner, 2000).

In certain embodiments, the expression cell has been engineered to carrya defective selectable marker gene, e.g. an antibiotic resistance genelike chloramphenicol or kanamycin, a fluorescent marker or a geneinvolved in a metabolic pathway of a sugar or an amino acid. In thiscase, the cartridge with the gene of interest carries the missing partof the marker gene, and by integration the marker gene restores itsfunctionality. By way of example, the cartridge carries the missing partof the marker gene at one of its ends, and is integrated directlyadjacent to the defective marker gene integrated in the genome, suchthat the fusion of the two fragments renders the marker gene completeand allows its functional expression. In the case of an antibioticresistance gene, the cells carrying the expression cassette areresistant against the specific antibiotic, in the case of a fluorescentmarker cells can be visualized by fluorescence, and in the case of ametabolic pathway gene, cells obtain the ability to metabolize therespective component (schematically shown in FIG. 11). The advantage ofthis embodiment is that only a short proportion of the marker gene ofthe cartridge needs to be synthesized, enabling shorter or smallerinsertion cartridges compared to prior art.

In certain embodiments, selection of positive clones (i.e. clones thatcarry the expression cassette) is carried out by correction (i.e.complementation) of an auxotrophy of the host cell. In such embodiments,as schematically shown in FIG. 8, a host cell is used that has amutation that has been chosen to allow selection of positivetransformant colonies in an easy way, e.g. a strain that has a deletionor mutation that renders it unable to synthesize a compound that isessential for its growth (such mutation being termed as “auxotrophicmarker”). For example, a bacterial mutant in which a gene of the prolinesynthesis pathway is inactivated, is a proline auxotroph. Such a strainis unable to synthesize proline and will therefore only be able to growif proline can be taken up from the environment, as opposed to a prolineprototroph which can grow in the absence of proline.

Any host cell having an auxotrophic marker may be used. Preferably,mutations in genes required for amino acid synthesis are used asauxotrophic markers, for instance mutations in genes relevant for thesynthesis of proline, leucine or threonine, or for co-factors likethiamine. According to the invention, the auxotrophy of host cells iscorrected by integration of the missing/defective gene as a component ofthe expression cartridge into the genome along with integration of thegene of interest. The thus obtained prototrophic cells can be easilyselected by growing them on a so-called “minimal medium” (prototrophicselection), which does not contain the compound for which the originalhost cell is auxotroph, thus allowing only positive clones to grow.

Prototrophic selection is independent of the integration locus. Theintegration locus for prototrophic selection may be any gene in thegenome or at the locus carrying the auxotrohic marker. The particularadvantage of prototrophic selection is that no antibiotic resistancemarker nor any other marker that is foreign to the host remains in thegenome after successful integration. Consequently, there is no need forremoval of said marker genes, providing a fast and simple cloning andselection procedure. Another advantage is that restoring the genefunction is beneficial to the cell and provides a higher stability ofthe system.

Alternatively, also shown schematically in FIG. 8, the marker gene thatis inserted into the genome together with the expression cartridge, maybe a metabolic gene that allows a particular selection mode. Such ametabolic gene may enable the cell to grow on particular (unusual) sugaror other carbon sources, and selection of positive clones can beachieved by growing cells on said sugar as the only carbon source.

As described above, during long term cultivation of bacteria, adaptiveevolution (Herring et al, 2006) may cause an enhanced mutation frequencyat the integration site during expression of the chromosomally encodedrecombinant protein. The use of an auxotrophic knockout mutant strain incombination with an expression cartridge complementing the lackingfunction of the mutant strain (thereby generating a prototroph strainfrom an auxotroph mutant) has the additional advantage that the restoredgene provides benefits to the cell by which the cell gains a competitiveadvantage such that cells in which adaptive evolution has occurred arerepressed. Thereby, a means of negative selection for mutated clones isprovided.

In some embodiments (in the case that the protein of interest allows fordetection on a single-cell or single-colony basis, e.g. by FACS analysisor immunologically (ELISA)), no marker gene is required, since positiveclones can be determined by direct detection of the protein of interest.

Other Features

The integration methods for obtaining the expression host cell are notlimited to integration of one gene of interest at one site in thegenome; they allow for variability with regard to both the integrationsite and the expression cassettes. By way of example, more than onegenes of interest may be inserted, i.e. two or more identical ordifferent sequences under the control of identical or differentpromoters can be integrated into one or more different loci on thegenome. By way of example, it allows expression of two differentproteins that form a heterodimeric complex. Heterodimeric proteinsconsist of two individually expressed protein subunits, e.g. the heavyand the light chain of a monoclonal antibody or an antibody fragment(Andersen and Reilly, 2004; Holliger and Hudson, 2005). Examples forother heterodimeric proteins are for instance CapZ (Remmert et al.,2000), Ras farnesyltransferase (Tsao and Waugh, 1997),platelet-activating factor acetylhydrolase Ib (Sheffield et al., 2001)or human DNA helicase II (Ochem et al., 1997). These two sequencesencoding the monomers may be present on one expression cartridge whichis inserted into one integration locus. Alternatively, these twosequences may be also present on two different expression cartridges,which are inserted independently from each other at two differentintegration loci. In any case, the promoters and the induction modes maybe either the same or different.

Although the invention allows plasmid-free production of a protein ofinterest, it does not exclude that in the expression host cell a plasmidmay be present that carries sequences to be expressed other than thegene of interest, e.g. the helper proteins and/or the recombinationproteins described above. Naturally, care should be taken that in suchembodiments the advantages of the invention should not be overruled bythe presence of the plasmid, i.e. the plasmid should be present at a lowcopy number and should not exert a metabolic burden onto the cell.

Genome-based expression of recombinant proteins provides the followingmajor advantages:

With respect to the construction procedure of the expression host, theadvantages are (i) a simple method for synthesis and amplification ofthe linear insertion cartridge, (ii) a high degree of flexibility (i.e.no limitation) with respect to the integration locus, (iii) a highdegree of flexibility with respect to selection marker and selectionprinciple, (iv) the option of subsequent removal of the selectionmarker, (v) the discrete and defined number of inserted expressioncartridges (usually one or two).

The expression system useful in the method of the invention may bedesigned such that it is essentially or completely free of phagefunctions.

Integration of one or more recombinant genes into the genome results ina discrete and pre-defined number of genes of interest per cell. In theembodiment of the invention that inserts one copy of the gene, thisnumber is usually one (except in the case that a cell contains more thanone genomes, as it occurs transiently during cell division), as comparedto plasmid-based expression which is accompanied by copy numbers up toseveral hundred. In the expression system used in the method of thepresent invention, by relieving the host metabolism from plasmidreplication, an increased fraction of the cell's synthesis capacity isutilized for recombinant protein production. Strong promoters, such asthe T7 system, can be applied without adverse effects on host metabolismby reduction of the gene dosage.

A particular advantage is that the expression system has no limitationswith regard to the level of induction. This means that the system cannotbe “over-induced” as it often occurs in plasmid-based systems, where PCNmay strongly increase upon induction (Teich et al., 1998; Wrobel andWegrzyn, 1998). A strong expression system in combination with a highand/or increasing gene dosage triggers a metabolic overload and leads toa dramatic loss of cell viability (Bently et al., 1990; Glick, 1995;Cserjan-Puschmann et al., 1999) and causes a significantly shortenedperiod of product formation and a loss of overall yield.

As mentioned above, plasmid-based expression systems have the drawbackthat, during cell division, cells may lose the plasmid and thus the geneof interest. Such loss of plasmid depends on several external factorsand increases with the number of cell divisions (generations). Thismeans that plasmid-based fermentations are limited with regard to thenumber of generations (in conventional fermentations, this number isapproximately between 20 and 50). In contrast, the genome-basedexpression system used in the method of the invention ensures a stable,pre-defined gene dosage for a practically infinite number of generationsand thus theoretically infinite cultivation time under controlledconditions (without the disadvantage of the occurrence of cells that donot produce the protein of interest and with the only limitation ofpotentially occurring natural mutations as they may occur in any gene).

In the case of chemically-inducible promoters, the invention providesthe particular advantage that the amount of inducer molecule, when e.g.added in a continuous mode according to Striedner et al (2003), isdirectly proportional to the gene dosage per cell, either constant overthe entire cultivation, or changing over cultivation time at pre-definedvalues. Thereby control of the recombinant expression rate can beachieved, which is of major interest to adjust the gene expression rate.

Since the genome-based expression system allows exact control of proteinexpression, it is particularly advantageous in combination withexpression targeting pathways that depend or rely on well-controlledexpression. In a preferred embodiment, the method of the inventionincludes secretion (excretion) of the protein of interest from thebacterial cytoplasm into the culture medium. The advantage of thisembodiment is an optimized and sustained protein secretion rate,resulting in a higher titer of secreted protein as compared to prior artsecretion systems. Secretion may occur by passive diffusion through thebacterial cell wall, or by active secretion into the periplasmic space,followed by diffusion or physico-chemical release (e.g. by using theStII signal sequence as described in U.S. Pat. No. 4,680,262 and in U.S.Pat. No. 4,963,495), or by direct translocation of the protein ofinterest from the cytoplasm into the culture medium (e.g. by methods asdescribed by Choi and Lee, 2004).

As described above, the invention allows to design simplified processes,improved process predictability and high reproducibility fromfermentation to fermentation. The process of the invention, employingthe expression system described above, is conducted in the fed-batch orin the semi-continuous or continuous mode (i.e. in a chemostat), wherebythe advantages of the genome-encoded expression system are optimallyexploited. There are no limitations with respect to process parameterssuch as growth rate, temperature and culture medium components, exceptas defined by the host cell's requirements and as pre-defined by theselected promoter.

Another advantage relates to the choice of the inducer molecule: Most ofthe available systems for high-level expression of recombinant genes inE. coli are lac-based promoter-operator systems (Makrides, 1996).Lactose, the precursor of the native inducer 1,6-allolactose, a naturaland inexpensive carbohydrate that is readily available in large amounts,has been considered as an alternative to IPTG, however, in the past,lactose was regarded as unsuitable for induction due to the problem ofinducer exclusion on glucose media and degradation in the cell. Theexpression system used in the invention allows a carbon-limitedcultivation with continuous supply of lactose as inducer (Striedner etal., 2003) and thus prevents the lactose exclusion, eliminates thedecreasing induction level observed during conventional pulse inductionwith lactose (Hoffman et al., 1995) and enables a tight expression ratecontrol even with lactose as inducer.

Importantly, the expression system used in the invention has theadvantage of providing a high yield of recombinant protein, both withregard to protein concentration per volume culture medium (i.e. thetiter) and with regard to protein content in the obtained biomass. Thisfeature makes the expression system used in the invention superiorcompared to prior art expression systems.

Furthermore, the invention offers the advantage that selection of theexpression host cell and/or the optimal design of the expressioncartridge, can be easily achieved in preliminary screening tests. By wayof example, in such preliminary screens a series of linear expressioncartridges that vary with respect to at least one element that has animpact on expression properties of the protein of interest (expressionlevel or qualitative features like biological activity), i.e. controlelements (e.g. promoter and/or polymerase binding site) and/or sequenceof the gene of interest (i.e. different codon usage variants) and/ortargeting sequences for recombination and/or any other elements on thecartridge, like secretion leaders, is constructed. The cartridgevariants are integrated into the genome of a pre-selected host cell andthe resulting expression host variants are cultivated, includinginduction of protein expression, under controlled conditions. Bycomparing protein expression, the host cell variant showing the mostfavourable results in view of an industrial manufacturing process isselected. In a variation of this pre-screening approach, instead ofdetermining the optimal expression cartridge, the optimal bacterialstrain may be identified by integrating identical cartridges into apanel of different host cells. Since the integration strategy has theadvantage of allowing integration of a discrete number of gene copies(e.g. only one) into the genome, pre-screening of various parameters maybe done without interference by plasmid replication or changes inplasmid copy number.

Manufacturing Scale Production

According to the invention, the term “cultivating” (or “cultivation”,also termed “fermentation”) relates to the propagation of bacterialexpression cells in a controlled bioreactor according to methods knownin the industry.

Manufacturing of recombinant proteins is typically accomplished byperforming cultivation in larger volumes. The term “manufacturing” and“manufacturing scale” in the meaning of the invention defines afermentation with a minimum volume of 5 L culture broth. Usually, a“manufacturing scale” process is defined by being capable of processinglarge volumes of a preparation containing the recombinant protein ofinterest, and yielding amounts of the protein of interest that meet,e.g. in the case of a therapeutic protein, the demands for clinicaltrials as well as for market supply. In addition to the large volume, amanufacturing scale method, as opposed to simple lab scale methods likeshake flask cultivation, is characterized by the use of the technicalsystem of a bioreactor (fermenter) which is equipped with devices foragitation, aeration, nutrient feeding, monitoring and control of processparameters (pH, temperature, dissolved oxygen tension, back pressure,etc.). The behaviour of an expression system in a lab scale method doesnot allow to predict the behaviour of that system in the complexenvironment of a bioreactor.

It was surprisingly found in the experiments of the invention that anexpression system based on genome integration is particularlyadvantageous for methods on a manufacturing scale (with respect to boththe volume and the technical system) in combination with a cultivationmode that is based on feeding of nutrients, in particular a fed-batchprocess or a continuous or semi-continuous process (e.g. chemostat).

Fed-Batch Cultivation

In certain embodiments, the method of the invention is a fed-batchprocess.

Whereas a batch process is a cultivation mode in which all the nutrientsnecessary for cultivation of the cells are contained in the initialculture medium, without additional supply of further nutrients duringfermentation, in a fed-batch process, after a batch phase, a feedingphase takes place in which one or more nutrients are supplied to theculture by feeding. The purpose of nutrient feeding is to increase theamount of biomass (so-called “High-cell-density-cultivation process” or“HCDC”) in order to increase the amount of recombinant protein as well.Although in most cultivation processes the mode of feeding is criticaland important, the present invention is not restricted with regard to acertain mode of feeding.

Feeding of nutrients may be done in a continuous or discontinuous modeaccording to methods known in the art. The feeding mode may bepre-defined (i.e. the feed is added independently from actual processparameters), e.g. linear constant, linear increasing, step-wiseincreasing or following a mathematical function, e.g. exponentialfeeding.

In a preferred embodiment, the method of the invention is a fed-batchprocess, wherein the feeding mode is predefined according to anexponential function. By applying an exponential feeding mode, thespecific growth rate μ of the cell population can be pre-defined at aconstant level and optimized with respect to maximum recombinant proteinexpression. Control of the feeding rate is based on a desired specificgrowth rate μ. When a defined medium, as described below, is used,growth can be exactly predicted and pre-defined by the calculation of abiomass aliquot to be formed based on the substrate unit provided.

In another preferred embodiment, an exponential feeding mode may befollowed, in the final stages of cultivation, by linear constantfeeding.

In another embodiment of the fed-batch process, linear constant feedingis applied. Linear constant feeding is characterized by the feeding rate(volume of feed medium per time unit) that is constant (i.e. unchanged)throughout certain cultivation phases.

In another embodiment of the fed-batch process, linear increasingfeeding is applied. Linear increasing feeding is characterized by afeeding rate of feed medium following a linear function. Feedingaccording to a linear increasing function is characterized by a definedincrease of feeding rate per a defined time increment.

In another embodiment of the fed-batch process of the invention, afeedback control algorithm is applied for feeding (as opposed to apre-defined feeding mode). In a feedback-controlled fed-batch process,the feeding rate depends on the actual level of a certain cultivationparameter. Cultivation parameters suitable for feedback-controlledfeeding are for instance biomass (and chemical or physical parametersderived thereof), dissolved oxygen, respiratory coefficient, pH, ortemperature (e.g. as described by Jahic et al., 2003). Another examplefor a feedback controlled feeding mode is based on the actual glucoseconcentration in the bioreactor (Kleman et al., 1991).

Continuous and Semi-Continuous Cultivation Mode

In another embodiment, bacterial cells carrying a genome-basedexpression cassette according to the present invention are cultivated incontinuous mode (chemostat). A continuous fermentation process ischaracterized by a defined, constant and continuous rate of feeding offresh culture medium into the bioreactor, whereby culture broth is atthe same time removed from the bioreactor at the same defined, constantand continuous removal rate. By keeping culture medium, feeding rate andremoval rate at the same constant level, the cultivation parameters andconditions in the bioreactor remain constant (so-called “steady state”).The specific growth rate μ can be pre-defined and is exclusively aresult of the feeding rate and the culture medium volume in thebioreactor. Since cells having one or more genome-based expressioncassettes are genetically very stable (as opposed to structurally andsegregationally instable plasmid-based expression systems, or expressionsystems which genome-inserted cassette relies on genomic amplification),the number of generations (cell doublings) of cells according to theinvention is theoretically unlimited, as well as, consequently,cultivation time. The advantage of cultivating a genetically stablegenome-based expression system in a continuous mode is that a highertotal amount of recombinant protein per time period can be obtained, ascompared to genetically unstable prior art systems. In addition, due tothe theoretically unlimited time of cultivation, continuous cultivationof cells according to the invention may lead to a higher total proteinamount per time period even compared to fed-batch cultivation processes.In Example 7, the high stability and productivity of a genome-basedexpression construct is shown.

Another preferred embodiment refers to semi-continuous cultivation ofcells. A semi-continuous cultivation process in the meaning of theinvention is a process which is operated in its first phase as afed-batch process (i.e. a batch phase followed by a feeding phase).After a certain volume or biomass has been obtained (i.e. usually whenthe upper limit of fermenter volume is obtained), a significant part ofcell broth containing the recombinant protein of interest is removedfrom the bioreactor. Subsequently, feeding is initiated again until thebiomass or volume of culture broth has again reached a certain value.This method (draining of culture broth and re-filling by feeding) can beproceeded at least once, and theoretically indefinite times.

Culture Medium

With regard to the type of the culture medium used in the fermentationprocess, there are no limitations. The culture medium may besemi-defined, i.e. containing complex media compounds (e.g. yeastextract, soy peptone, casamino acids), or it may be chemically defined,without any complex compounds.

Preferably, a “defined medium” is used. “Defined” media (also termed“minimal” or “synthetic” media) are exclusively composed of chemicallydefined substances, i.e. carbon sources such as glucose or glycerol,salts, vitamins, and, in view of a possible strain auxotrophy, specificamino acids or other substances such as thiamine. Most preferably,glucose is used as a carbon source. Usually, the carbon source of thefeed medium serves as the growth-limiting component which controls thespecific growth rate.

In the methods of the invention, significantly higher yields areobtained, because growth of bacteria and a high, but physiologicallytolerable recombinant gene expression rate can be maintained during thewhole production process.

Induction Mode

As described above, in a most preferred embodiment of the invention, theprotein of interest is under control of an “inducible” or “controllable”promoter.

There is no limitation as regards the mode by which induction of proteinexpression is performed. By way of example, the inductor can be added asa singular or multiple bolus or by continuous feeding, the latter beingalso known as “inductor feed(ing)”. There are no limitations as regardsthe time point at which the induction takes place. The inductor may beadded at the beginning of the cultivation or at the point of startingcontinuous nutrient feeding or after (beyond) the start of feeding.Inductor feeding may be accomplished by either having the inductorcontained in the culture medium or by separately feeding it.

The advantage of inductor feeding is that it allows to control inductordosage, i.e. it allows to maintain the dosage of a defined or constantamount of inductor per constant number of genes of interest in theproduction system. For instance, inductor feeding allows an inductordosage which is proportional to the biomass, resulting in a constantratio of inductor to biomass. Biomass units on which the inductor dosagecan be based, may be for instance cell dry weight (CDW), wet cell weight(WCW), optical density, total cell number (TCN; cells per volume) orcolony forming units (CFU per volume) or on-line monitored signals whichare proportional to the biomass (e.g. fluorescence, turbidity,dielectric capacity, etc.). Essentially, the method of the inventionallows the precise dosage of inductor per any parameter or signal whichis proportional to biomass, irrespective of whether the signal ismeasured off-line or on-line. Since the number of genes of interest isdefined and constant per biomass unit (one or more genes per cell), theconsequence of this induction mode is a constant dosage of inductor pergene of interest. As a further advantage, the exact and optimum dosageof the amount of inductor relative to the amount of biomass can beexperimentally determined and optimized, as demonstrated in Example 8and FIG. 16 and FIG. 17.

It may not be necessary to determine the actual biomass level byanalytical methods. For instance, it may be sufficient to add theinductor in an amount that is based on previous cultivations (historicalbiomass data). In another embodiment, it may be preferable to add theamount of inductor per one biomass unit as theoretically calculated orpredicted. For instance, it is well known for feeding-based cultivations(like fed-batch or continuous) that one unit of the growth-limitingcomponent in the feed medium, usually the carbon source, will result ina certain amount of biomas. As an example, 1 g glucose asgrowth-limiting substrate will result in approximately 0.33 g cell dryweight (also expressed by the substrate yield coefficient Y_(X/S)=0.33).Consequently, a defined dosage of inductor per gene of interest may alsobe achieved by the defined dosage of inductor per unit growthlimiting-component, since a certain unit of growth limiting componentresults in a defined unit of biomass, and a defined unit of biomasscontains a defined number of molecules of proteins of interest accordingto the method of the invention.

The concentration of IPTG may be in the range of 0.1-30 μg per g CDW,preferably, it is in the range of 0.5-20 μg per g CDW.

In Example 8, the maximum inductor dosage in the feed medium is 20 μmolIPTG per g CDW, which is equivalent to 6.6 μmol IPTG per g glucose,under the assumption of a substrate yield coefficient of Y_(X/S)=0.33.Consequently, since the glucose concentration in the feeding medium is128 g/L, the IPTG concentration in the same medium was 844 μM.

As an essential advantage, feeding limiting amounts of inductor preventsmetabolic load and reduces stress in favour of maximizing the capacityof protein synthesis.

The ratio of inductor per biomass (or per gene or per unitgrowth-limiting substrate) may not necessarily be constant. It may alsobe linear increasing, linear decreasing, increasing or decreasingaccording to exponential or other mathematical functions, etc. Theessential feature according to the invention is that the value ofinductor dosage per gene of interest is defined.

In certain embodiments, the method of the invention is a fed-batchprocess, wherein the inductor is present in the batch medium from startof cultivation.

The mode of induction of expression can also be constitutive, whichmeans that induction is not triggered chemically or by other stimuli,but that it is permanent from start of cultivation. Constitutiveinduction is the preferred induction mode for continuous cultivation,but also useful for fed-batch cultivation.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1: Fed-batch cultivation employing genome-encoded expression systemHMS174 (DE3)<Cam:T7:6×His-NproEddieGFPmut3.1> for production of aninsoluble autoprotease fusion protein (ES4).

FIG. 2: Fed-batch cultivation employing plasmid-encoded expressionsystem HMS174 (DE3) (pET30a 6×His-NproEddie-GFPmut3.1) for production ofan insoluble autoprotease fusion protein (ES1).

FIG. 3: Fed-batch cultivation employing genome-encoded expression systemBL21 (DE3)<Cam:T7:6×His-NproEddieGFPmut3.1> (ES5).

FIG. 4: Fed-batch cultivation employing plasmid-encoded expressionsystem BL21 (DE3) (pET30a 6×His-Npro-GFPmut3.1) for production of aninsoluble autoprotease fusion protein (ES2).

FIG. 5: Fed-batch cultivation employing genome-encoded expression systemHMS174 (DE3TN7::<Kan:T7 GFPmut3.1> for production of the soluble greenfluorescent protein (ES6).

FIG. 6: Fed-batch cultivation employing plasmid-encoded expressionsystem HMS174 (DE3) (pET30a GFPmut3.1) for production of the solublegreen fluorescent protein (ES3).

FIG. 7: Selection by insertion of an antibiotic resistance marker,optionally followed by removal (excision) of marker.

FIG. 8: Selection by providing a non-essential metabolic gene or aprototrophic complementation gene as marker.

FIG. 9: Insertion of expression cartridge at the genomic site ofrecombination proteins, thereby removing them.

FIG. 10: Insertion of expression cartridge at the site of a marker gene,and selection for disappearance of marker phenotype.

FIG. 11: Selection by completion of the missing part of a non-functionalmarker to generate a functional, selectable marker.

FIG. 12: Fed-batch cultivation employing plasmid-encoded expressionsystem HMS174 (DE3) for production of the soluble hSOD (ES7).

FIG. 13: Fed-batch cultivation employing genome-encoded expressionsystem HMS174 (DE3) for production of the soluble hSOD (ES8).

FIG. 14: Fed-batch cultivation employing genome-encoded expressionsystem BL21 (DE3) for production of the soluble hSOD (ES9).

FIG. 15: Expression of soluble GFPmut3.1 with E. coli HMS174 (DE3) inthe continuous mode (chemostat)) (ES6).

FIG. 16: Fed-batch cultivation employing genome-encoded expressionsystem BL21 (DE3) in combination with controlled inductor dosage forproduction of the soluble GFPmut3.1.

FIG. 17: Standardized average product formation rate (qP) of solubleGFPmut3.1 obtained from fed-batch cultivation employing genome-encodedexpression system BL21 (DE3), in correlation with the concentration ofinductor.

EXAMPLES Example 1

Generation of Bacterial Strains and Description of Recombinant Proteins

Experiments are performed either with the recA⁻ K12 strain E. coliHMS174 (DE3) or with the B-strain BL21 (DE3) provided by Novagen. Thedesignation (DE3) indicates that the hosts are lysogens of λ DE3prophage, carrying a chromosomal copy of the T7 RNA polymerase geneunder control of the lacUV5 promoter, making these strains suitable forprotein expression using T7 or T7 lac promoters (Studier and Moffat,1986; Studier et al., 1990). Integration of linear DNA cartridges intothe bacterial chromosome is done in E. coli MG1655 carrying the Redhelper plasmid pKD46 as described by Datsenko and Wanner (2000). Thelinear expression cartridge is integrated into the attTn7 site, which isbetween the glmS gene and the pstS gene at the genome of E. coli. PCRverification of cassettes on the genome is done by combining externalprimers annealing to the genome and internal primers. By P1 transduction(Sternberg and Hoess, 1983; and Lennox, 1955), the recombinantchromosomal section is transferred into the expression hosts HMS174(DE3) and BL21 (DE3). To act as a recipient, the recA deficient strainMS174 (DE3) is used that contains the temperature-sensitive pTSA29-recAhelper plasmid, providing the RecA protein (Phillips, 1998).

Reference experiments with prior art plasmid-based expression of targetproteins are performed with pET30a and pET11a plasmids from Novagen (pETSystem manual, 11^(th) edition).

To show the potential of the expression system of the invention, asoluble protein (Green Fluorescent Protein GFP; Shimomura et al., 1962))and an inclusion body forming protein (Npro) are used in theexperiments. GFPmut3.1 is a FACS-optimised variant (Cormack et al.,1996) with substitutions S65G and S72A for improved folding andexcitation at 488 nm. The inclusion body forming protein 6×His-N^(pro)Eddie-GFPmut3.1 is a fusion of the above-described GFPmut3.1 and N^(pro)Eddie, a modified variant (described in WO 2006/113959) of the nativeN^(pro) autoprotease from classical swine fever virus (Thiel et al.,1991). N^(pro) is a non-structural protein consisting of 168 amino acids(apparent Mr of 23000), which cleaves itself co-translationally from thenascent polyprotein, thereby generating the correct N-terminus of thecapsid protein C (Wiskerchen et al., 1991; Stark et al., 1993).

TABLE 1 List of generated expression systems. Designation of plasmid orintegration cartridge/ target protein Abbreviation (underlined: ofexpression Location of protein the respective Described in system Straingene of interest system is encoding) Example ES1 HMS174(DE3) Plasmid(pET30a 6xHis- Example 3 N^(pro)Eddie-GFPmut3.1) ES2 BL21(DE3) Plasmid(pET30a 6xHis- Example 4 N^(pro)Eddie-GFPmut3.1) ES3 HMS174(DE3) Plasmid(pET11a GFPmut3.1) Example 5 ES4 HMS174(DE3) Genome <Cam:T7:6xHis-Example 3 N^(pro)Eddie-GFPmut3.1> ES5 BL21(DE3) Genome <Cam:T7:6xHis-Example 4 N^(pro)Eddie-GFPmut3.1> ES6 HMS174(DE3) Genome <Kan:T7GFPmut3.1> Example 5 and Example 7 ES7 HMS174(DE3) Plasmid (pET11ahSOD)Example 6 ES8 HMS174(DE3) Genome <Cam:T7:hSOD> Example 6 ES9 BL21(DE3)Genome <Cam:T7:hSOD> Example 6 ES10 BL21(DE3) Genome <Cam:T7:GFPmut3.1>Example 8

Example 2

Cultivation Mode and Process Analysis

The cells are grown either in a 7 L (5 L net volume, 2.5 L batch volume)or in a 20 L (12 L net volume, 8 L batch volume) computer-controlledbioreactor (MBR; Wetzikon, CH) equipped with standard control units. ThepH is maintained at a set-point of 7.0±0.05 by addition of 25% ammoniasolution (ACROS Organics), the temperature is set to 37° C.±0.5° C. Inorder to avoid oxygen limitation, the dissolved oxygen level isstabilized above 30% saturation by stirrer speed and aeration ratecontrol. The content of O₂ and CO₂ in the outlet air is determined by aHartmann and Braun Advanced Optima gas analyzer. Dielectric capacity andconductivity are measured with the Biomass monitor, model 214M (AberInstruments, Aberystwyth, UK) set. Fluorescence measurements areperformed using a multi-wavelength spectrofluorometer specially designedfor online measurements in an industrial environment, the BioView®(DELTA Light & Optics, Lyngby, Denmark). Foaming is suppressed byaddition of antifoam suspension (Glanapon 2000, Bussetti, Vienna) with aconcentration of 1 ml/l feed medium. For inoculation, a deep frozen(−80° C.) working cell bank vial, is thawed and 1 ml (optical densityOD₆₀₀=1) is transferred aseptically to the bioreactor. Feeding isstarted when the culture, grown to a bacterial dry matter of 12.5 g in2.5 L batch medium (or 30 g in 4 L batch medium), entered stationaryphase. Fed-Batch regime with an exponential substrate feed is used toprovide a constant growth rate of 0.1 h⁻¹ during 4 doubling times. Thesubstrate feed is controlled by increasing the pump speed according tothe exponential growth algorithm, x=x_(o)·e^(μt), with superimposedfeedback control of weight loss in the substrate tank (Cserjan-Puschmannet al., 1999). The feed medium provided sufficient components to yieldanother 202 g (or 450 g) of bacterial dry matter.

Induction

Induction is performed in a conventional mode by a single pulse directlyinto the bioreactor. The supplied amount of IPTG is calculated to set aconcentration of 1 μmol IPTG at the end of the process in order to gaina fully induced system.

Media Composition

The minimal medium used in this study contains 3 g KH₂PO₄ and 6 gK₂HPO₄*3H₂O per liter. These concentrations provide the required buffercapacity and serve as P and K source as well. The other components areadded in relation of gram bacterial dry matter to be produced: sodiumcitrate (trisodium salt*2H₂O; ACROS organics) 0.25 g, MgSO₄*7H₂O 0.10 g,CaCl₂*2H₂O 0.02 g, trace element solution 50 μl and glucose*H₂O 3 g. Inall experiments with expression systems ES6, ES7 and ES8, medium issupplemented with CuCl₂*2H₂O 4 mg and ZnSO₄*7H₂O 3.2 mg per grambacterial dry matter. To accelerate initial growth of the population,the complex component yeast extract 0.15 g is added to the minimalmedium to obtain the batch medium. For the feeding phase 2.5 L ofminimal medium are prepared according to the amount of biological drymatter 202 g (or 450 g) to be produced in the feeding phase, wherebyP-salts are again added per liter. Trace element solution: prepared in 5N HCl (g/L): FeSO₄*7H₂O 40.0, MnSO₄*H₂O 10.0, AlCl₃*6H₂O 10.0, CoCl₂(Fluka) 4.0, ZnSO₄*7H₂O 2.0, Na₂MoO₂*2H₂O 2.0, CuCl₂*2H₂O 1.0, H₃BO₃0.50.

Offline Analysis

Optical density (OD) is measured at 600 nm. Bacterial dry matter isdetermined by centrifugation of 10 ml of the cell suspension,re-suspension in distilled water followed by centrifugation, andre-suspension for transfer to a pre-weighed beaker, which is then driedat 105° C. for 24 h and re-weighed.

The progress of bacterial growth is determined by calculating the totalamount of biomass (total bacterial dry matter BDM; also termed cell dryweight CDW).

Total Cell Number (TCN) and percentage of dead cells (DC) are determinedusing flow cytometric methods. All measurements are performed with aFACSCalibur flow cytometer (four-color system; Becton Dickinson),equipped with an air-cooled laser providing 15 mW at 488 nm and thestandard filter setup. Samples are spiked with a known amount offluorescent counting beads (Becton Dickinson, USA) so that the absolutecell number could be back-calculated.

The content of recombinant protein Npro-Eddie-GFPmut3.1 is determined bya combination of ELISA and electrophoretic protein quantification usingthe Agilent Bioanalyser. Soluble recombinant product is quantified viaGFP-ELISA according to Reischer et al. (2004), while the recombinantproduct in the inclusion bodies is determined with the AgilentBioanalyser 2100, using the Protein 200 LabChip® Kit.

The content of recombinant GFP is quantified via GFP-ELISA according toReischer et al. (2004). The content of hSOD is quantified via SOD ELISAaccording to Bayer et. al (1990).

Plasmid-containing cells but also cells carrying the integrationcassette are determined by cultivation on LB-agar plates and on platescontaining 100 mg/ml Kanamycin respectively 100 mg/ml Ampicillin and bycounting colony forming units (CFU) after 24 h.

Example 3

Expression of 6×His-NproEddie-GFPmut3.1 as Insoluble Inclusion Body withE. coli HMS174 (DE3) in the Fed-Batch Mode

Expression systems ES1 and ES4 listed in Table 1 are used in order tocharacterize the behaviour of the expression system of the invention(ES4) for production of insoluble target proteins (i.e. as inclusionbodies), and to evaluate the economic potential compared to plasmidbased expression systems (ES1). The experiment with the expressionsystem of the invention shows high but physiologically tolerable geneexpression rates. The product formation can be maintained for more than20 hours. Induction is performed as single pulse one doubling past feedstart in order to gain a fully induced system (FIG. 1).

In contrast to these results, the gene expression rate triggered by theplasmid based expression system overstrains the metabolic capacities ofthe cells and lead to the breakdown of the cellular system four hourspast induction, performed as single IPTG pulse tree doublings past feedstart (FIG. 2). Expression system ES4 according to the invention showeda slightly higher volumetric product yield (Table 2). As can be seenfrom these experiments, the expression system of the invention offersthe advantage that process control and defined conditions can bemaintained during the whole period of product formation.

TABLE 2 Summary of results from Example 3 (Comparison of genome-encodedand plasmid-encoded production of the protein 6xHis-NproEddie-GFPmut3.1with the host strain HMS174(DE3). Genome Plasmid- encoded encoded (ES1)(ES4) Total yield of bacterial dry matter 158 486 (BDM) [g] Deviation ofBDM yield in % 20 30 from theoretically calculated BDM yield Specificcontent of recombinant 144 156 protein [mg/gBDM] Volumetric yield(titer) of 5.0 4.7 recombinant protein [g/L]

Example 4

Expression of 6×His-NproEddie-GFPmut3.1 as Insoluble Inclusion Body withE. coli BL21 (DE3) in the Fed-Batch Mode

In order to prove the general applicability of the expression system ofthe invention, strain E. coli BL21 (DE3) is used as expression host. ES5of Table 1 is cultivated in the 5 L scale according to the proceduredescribed above. The results of this system (FIG. 3; induction isperformed as single pulse one doubling past feed) are better than theresults with the K12 strain HMS174 and, compared to the results obtainedwith the plasmid-based ES2 (FIG. 4); induction is performed as singlepulse tree doublings past feed start.), the system of the inventionshows significantly higher product yields and higher process stabilitythan the plasmid-based system (Table 3). The plasmid-based system ofstrain BL21 shows the same characteristics as described with strainHMS174, and too high product formation rates lead to the breakdown ofthe system and loss of process control.

TABLE 3 Summary of results from Example 4 (Comparison of genome-encodedand plasmid-encoded production of the protein 6xHis-NproEddie-GFPmut3.1with the host strain BL21(DE3). Genome Plasmid- encoded encoded (ES5)(ES2) Total yield of bacterial dry matter 144 486 (BDM) [g] Deviation ofBDM yield in % from 25 15 theoretically calculated BDM yield Specificcontent of recombinant 256 192 protein [mg/gBDM] Volumetric yield(titer) of 8.2 6.6 recombinant protein [g/L]

Example 5

Expression of GFPmut3.1 as Soluble Protein with E. coli HMS174 (DE3) inthe Fed-Batch Mode

Expression system ES3 and ES6 listed in Table 1 are used in order tocharacterise the behaviour of the expression system of the invention forproduction of soluble target proteins (the other expression systems ofTable 1 form insoluble protein aggregates, i.e. inclusion bodies). Inorder to exclude effects triggered by different cell densities,identical cultivations with induction one doubling past feed start areperformed, although the results of the other experiments demonstratedthat the plasmid based systems triggers too high expression rates andcell activity cannot be maintained with this system. To cope withrequired comparability there is a third column in Table 4 called“plasmid-encoded optimized” which means that the results of ES2“plasmid-encoded” are extrapolated to an experimental setup simulatinginduction at one doubling past start of feed. As can be derived fromFIG. 5 and FIG. 6 (induction is performed as single pulse tree doublingspast feed start) as well as from Table 4, the expression system of theinvention proved to be more efficient. This is especially demonstratedby a higher specific and volumetric yield of recombinant protein and abetter conversion rate of nutrients into biomass (i.e. a higher BDMyield or a lover deviation rate from the theoretical value).

TABLE 4 Summary of results from Example 5 (Comparison of genome-encodedand plasmid-encoded production of the protein GFPmut3.1 with the hoststrain HMS174(DE3). Plasmid- Genome- Plasmid- encoded, encoded encodedoptimized (ES6) (ES3) (ES3) Total yield of bacterial 391 129 356 drymatter (BDM) [g] Deviation of BDM yield in % from theoretically 19 26 26calculated BDM yield Specific content of 213 141 141 recombinant protein[mg/gBDM] Total rec. protein yield 101 18 50 [g] Volumetric proteinyield 8.95 2.93 4.43 (titer) [g/L]

Example 6

Expression of Soluble hSOD E. coli HMS174 (DE3) and BL21 (DE3) inFed-Batch Mode

Recombinant human superoxide dismutase (hSOD) is produced byplasmid-based and by genome-based expression with E coli HMS174 (DE3)(ES7, ES8) and additionally by genome-based expression using E. coliBL21 (DE3) (ES9). Human SOD is selected as third model protein becausethis enzyme is described as highly interactive in the bacterial cytosol,thereby representing a well suited protein candidate to investigaterobustness and performance of the production method using genome-basedexpression. In the experiment with ES7 (FIG. 12) the amount ofbiological dry matter to be produced is pre-defined to 225 g BDM andinduction is performed 2.5 doublings past feed start. In thecultivations with expression system ES8 (FIG. 13) and ES9 (FIG. 14) thecalculated BDM is pre-defined to 360 g BDM and induction is performed atone doubling past start of feed. In order to allow proper comparison ofES7 and ES8, calculated values for total yield and the volumetric yieldobtained in the experiment with ES7 are used as this experiment is runat a lower cell density and a smaller volume. The results in Table 5show that the yields obtained with genome based ES8 are about 3-foldhigher and the behavior during production is more robust and stable(indicated by a smaller difference between BDM calculated and BDM real).Even though there is a slight BDM deviation from the calculated course,growth can be maintained during the whole process indicating that genomeencoded hSOD expression does not exceed the metabolic capacity of thecell which is in contrast to plasmid-encoded SOD expression. The resultsdelivered by the experiment with genome-based ES9 (Table 5 column 4)show a further increase in yield of approximately 20% which means thatthe BL21 (DE3) strain is even better suited for production of hSOD thanthe K12 strain.

TABLE 5 Summary of results from Example 5 (Comparison of genome-encodedand plasmid-encoded production of the protein hSOD with the host strainHMS174(DE3), and production of genome-encoded hSOD with strainBL21(DE3). Values marked with * are calculated values to ensurecomparability (due to the fact that the plasmid encoded system is run ata lower cell density). HMS174(DE3) HMS174(DE3) BL21(DE3) Plasmid-Genome- Genome- encoded encoded encoded ES7 ES8 ES9 Total yield of 140360 391 biomass dry matter (BDM) [g] Deviation of 36 26 20 BDM yield in% from theoretically calculated (pre- defined) BDM yield Specificcontent 64 190 210 of recombinant protein [mg/gBDM] Total rec. protein8.9 50.6 60.3 yield [g] (16.55*) Volumetric 0.92 4.5 5.4 protein yield(1.48*) (titer) [g/L]

Example 7

Expression of Soluble GFPmut3.1 with E. coli HMS174 (DE3) in theContinuous (Chemostat) Mode

In order to verify the genetic stability of a genome-based (i.e.plasmid-free) expression system, a continuous cultivation experiment(chemostat) is conducted with the expression system ES6. A dilution rateof 0.1 h⁻¹ and a cell density of 10 g BDM per liter are set and thedefined medium according to Example 2 is used for the main culture. Theresult of this cultivation is shown in FIG. 15. In the first part of theexperiment the cells are grown in a non-induced state. The non-inducedstate of the main culture is maintained for more than 42 doublingswithout any detectable changes in the behavior of the subcultures. Thefraction of producing cells is kept more or less constant between 75 to95% (squares FIG. 15). Subsequently, close to 300 hours of feeding, theculture is fully induced with IPTG and about 5 hours past induction theonline fluorescence approximates the maximal level obtainable in thisexperimental setup. According to Reischer et al (2004) the specificcellular content of GFP is calculated to be approximately 140 to 170 mgGFP per g BDM. The cells are maintained in this induced state for morethan 13.5 doublings. The course of the online fluorescence during thisperiod shows rfu range between 6000 and 7000 units. These results showthat the system stability of the genome based expression host is veryhigh even for prolonged generations in the induced state. In contrast toplasmid-based systems, very high expression rates can be maintained withthe systems based on genome integration which represents a clear benefitfor the development of continuous manufacturing processes.

Example 8

Improved Induction Control by Pre-Defining Inducer Concentration

Tight induction control of recombinant gene expression is a veryimportant aspect in bioprocessing. Contrary to plasmid-based pET/T7systems with their high and varying gene dosage (plasmid copy number),the genome-based (plasmid free) T7 system used in the method of theinvention strictly features a defined number of gene copies (one ormore) of the target gene. Due to this fact, the expression rate ofgenome based systems can be controlled more efficient. To demonstratethis benefit, a series of fed-batch cultivations with ES10 applyingdifferent induction levels (i.e. inducer concentrations) is conducted.Induction is performed by a single bolus of IPTG into the bioreactor toset an initial ratio of inducer per cell dry weight (CDW), followed bycontinuously feeding of IPTG to keep a defined concentration at aconstant level. The ratios of IPTG to CDW are as follows: 0.5, 0.75,1.0, 2.0 μmol gCDW⁻¹, up to a level of 20 μmol gCDW⁻¹ which certainlyresults in a full titration of lac-repressor molecules (i.e. in thehighest possible induction level). The off-line measured specificcellular fluorescence (spec. F) which is strongly correlated to thespecific content of GFP (Reischer et. al 2004) is used to monitorproduct accumulation during induction and to calculate the specificproduct formation rate (qP_(F)) as follows:

${qP}_{F} = \frac{\left( {{{spec}.F_{2}} - {{spec}.F_{1}}} \right)}{\left( \frac{{CDW}_{1} + {CDW}_{2}}{2} \right)*\left( {t_{2} - t_{1}} \right)}$

The applied range of IPTG to CDW ratios results in cellular fluorescencelevels from 2000 to 10000 rfu's g⁻¹ CDW at the end of the process whichimpressively proves the efficiency of the transcription tuning strategy(FIG. 16). The qP_(F) of the fully induced system (20 μmol IPTG/gCDW) isfixed as maximum level of 100% and the average qP_(F) values obtained inthe experiments with limited induction are strongly correlated with theinduction level (inducer concentration) following a logarithmic trend(FIG. 17 shows the standardized pP). These results imply that low levelinduction is more sensitive to changes in the IPTG/CDW ratio andrequires tightly controlled inducer feed regimes based on real timeestimated CDW. Furthermore the IPTG/CDW ratio which is required for fullinduction of the system can be estimated by extrapolation of the trendin FIG. 17 and will range between 3-5 μmol g⁻¹, which is substantiallylower than the 20 μmol g⁻¹ used in the experiment. Each applied IPTG/CDWratio results in a corresponding qP_(F) level with a certain band-width,as follows:

Induction level (IPTG 0.5 0.75 1 2 concentration in μmol/gCDW) Maximumlevel of specific 500 800 1000 1200 product formation rate qP_(F)(rfu/gh)

As a conclusion, this example shows that a method based on genome-basedexpression according to the invention enables a better controllabilityof product yield and product formation rate.

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The invention claimed is:
 1. A method for plasmid-free production of aheterologous protein of interest on a manufacturing scale, consisting ofthe steps of a) cultivating expression host cells that are E. coli cellsin a culture medium, on a manufacturing scale in a bioreactor with aminimum volume of 5 L, said cultivating comprising a mode that employsadding a feeding medium, wherein said mode is selected from fed-batchmode, semi-continuous mode, and continuous mode, wherein one or twocopies of a DNA construct are integrated in the chromosome of the hostcells, and wherein said DNA construct consists of a DNA sequenceencoding the heterologous protein of interest under the control of apromoter that enables expression of the heterologous protein from theintegrated one or two copies of said DNA construct, a ribosome bindingsite, two terminally flanking regions homologous to a genomic region ofthe host cells that enable homologous recombination, and optionally oneor more of the components selected from gene sequences coding forantibiotic selection markers, prototrophic selection markers orfluorescent markers, genes coding for metabolic markers, and genes thatimprove protein expression, and flippase recognition target sites, andwherein the expression host cells contain a T7 RNA polymerase gene inthe chromosome and wherein said promoter is a T7 promoter, wherein theone or two copies of said DNA constructs are maintained in thechromosome of the cultivated expression host cells, and wherein the DNAsequence encoding the heterologous protein of interest is not present ina plasmid; b) harvesting the heterologous protein of interest expressedby the expression host cells; and c) isolating and purifying theheterologous protein of interest.
 2. The method of claim 1, wherein theDNA construct carries, as an additional element, a T7 RNA polymerasegene.
 3. The method of claim 1, wherein said E. coli cells are selectedfrom the strains BL21(DE3), HMS174(DE3) or their derivatives.
 4. Themethod of claim 1, wherein said E. coli cells are non-lysogenic.
 5. Themethod of claim 1, wherein said expression host cells contain said DNAconstruct integrated at an attachment site.
 6. The method of claim 5,wherein said attachment site is the attTn7 site.
 7. The method of claim1, wherein said expression host cells contain said DNA constructintegrated at the site of a DNA marker sequence that is contained in thechromosome of the host cell.
 8. The method of claim 7, wherein saidmarker DNA sequence encodes a protein that provides antibioticresistance.
 9. The method of claim 7, wherein said marker DNA sequenceencodes a fluorescent protein.
 10. The method of claim 1, wherein saidDNA construct contains, as an additional element, a marker sequenceencoding a marker protein.
 11. The method of claim 10, wherein saidmarker protein is a protein which complements an auxotrophic mutation ofthe host cell, thereby rendering the expression host cell prototrophic.12. The method of claim 10, wherein said marker DNA sequence encodes aprotein that confers an antibiotic resistance.
 13. The method of claim10, wherein said marker DNA sequence encodes a fluorescent protein. 14.The method of claim 1, wherein the step of cultivating is in thefed-batch mode with a predefined feeding mode.
 15. The method of claim1, wherein the step of cultivating is in the fed-batch mode with afeedback-controlled feeding mode.
 16. The method of claim 15, whereinthe heterologous protein of interest is secreted from the cytoplasm ofthe expressing host cells into the culture medium.
 17. The method ofclaim 1, wherein the expression of said protein of interest is inducedby the presence of an inductor.
 18. The method of claim 17, wherein theinductor is added in a continuous manner.
 19. The method of claim 17,wherein the inductor is IPTG.
 20. The method of claim 17, wherein theinductor is lactose.
 21. The method of claim 17, wherein the inductor ispresent from the beginning of the cultivating step a).
 22. The method ofclaim 1, wherein the heterologous protein of interest is a heterodimerand wherein the DNA molecules encoding the two monomers are present onthe same or on different DNA constructs.